(CNN) -- One of the worst pandemics in human history, the Black Death, along with a string of plague outbreaks that occurred during the 14th to 19th centuries, was spread by human fleas and body lice, a new study suggests. "This study uses mathematical modeling to try and determine how exactly the plague spread in Europe during the pandemic known as the 'Black Death'," said Raina MacIntyre, professor of Infectious Diseases Epidemiology at UNSW Sydney, READ MORE